Analog to Digital Converters

What is Analog to Digital Converters?

There are many methods to convert analog signals to digital signals. These converters find more applications as an intermediate device to convert the signals from analog to digital form, displays output on LCD through a microcontroller. The objective of an A/D converter is to determine the output signal word corresponding to an analog signal. Now we are going to see an ADC of 0804. It is an 8-bit converter with 5V of power supply. It can take only one analog signal as input.

The digital output varies from 0-255. ADC needs a clock to operate. The time taken to convert the analog to digital value depends on the clock source. An external clock can be given to CLK IN pin no.4. A suitable RC circuit is connected between the clock IN and clock R pins to use the internal clock. Pin2 is the input pin – High to low pulse brings the data from internal register to the output pins after conversion. Pin3 is a Write – Low to high pulse is given to external clock. Pin11 to 18 are data pins from MSB to LSB.

Video on Circuit Diagram of Analog to Digital converter ADC

Normal operation is that when the logic high, input has been clocked through the 8-bit shift register, completing the SAR search, on the next clock pulse, the digital word is transferred to the 3-state output. The output of the interrupt is inverted to provide an INTR output that is high during conversion and low when the conversion is completed. When a low is at both CS and RD, an output is applied to the DB0 through DB7 outputs and the interrupt is reset. When either the CS or RD inputs return to a high state, the DB0 through DB7 outputs are disabled (returned to the high-impedance state). The interrupt flip-flop remains reset.

2 Examples of ADC:

1.      ADC0808:

ADC0808 is a converter which has 8 analog inputs and 8 digital outputs.  ADC0808 allows us to monitor up to 8 different transducers using only a single chip. This eliminates the need for external zero and full scale adjustments.

ADC0808 is a monolithic CMOS device, offers high speed, high accuracy, minimal temperature dependence, excellent long-term accuracy and repeatability and consumes minimal power. These features make this device ideally suited to applications from process and machine control to consumer and automotive applications. The pin diagram of ADC0808 is shown in figure below:

ADC0808Features:

  • Easy interface to all microprocessors
  • No zero or full-scale adjust required
  • 8-channel multiplexer with address logic
  • 0V to 5V input range with single 5V power supply
  • Outputs meet TTL voltage level specifications
  • Carrier chip package with 28-pin

Specifications:

  • Resolution: 8 Bits
  • Total Unadjusted Error: ±½ LSB and ±1 LSB
  • Single Supply: 5 VDC
  • Low Power: 15 mW
  • Conversion Time: 100 μs

Generally, the ADC0808 input which is to be changed over to digital form can be selected by using three address lines A, B, C which are pins 23, 24 and 25. The step size is chosen dependent upon set reference value. Step size is the change in analog input to cause a unit change in the output of ADC. ADC0808 needs an external clock to operate unlike ADC0804 which has an internal clock.

The continuous 8-bit digital output corresponding to instantaneous value of analogue input. The most extreme level of input voltage must be reduced proportionally to +5V.

The ADC 0808 IC requires clock signal of typically 550 kHz, ADC0808 is used to convert the data into digital from required for the microcontroller.

Application of ADC0808:

The ADC0808 has got many applications; here we have given some application on ADC:

From the below circuit the clock, start and EOC pins are connected to microcontroller. Generally, we have 8 inputs; here we are using only 4 inputs for the operation.

  • LM35 temperature sensor is using which is connected to fist 4 inputs of the analog to digital convertor IC. The sensor has got 3 pins i.e., VCC, GND and output pins, when the sensor heated the voltage at output increases.
  • The address lines A, B, C are connected to microcontroller for the commands. In this the interrupt follows the low to high operation.
  • When the start pin is held high no conversion begins, but when the start pin is low the conversion will start within 8 clock periods.
  • The point when the conversion is completed the EOC pin goes low to indicate the finish of conversion and data ready to be picked up.

The output enable (OE) is then raised high. This enables the TRI-STATE outputs, allowing the data to be read.

ADC0808 Circuit

2.      ADC0804:

We already know that analog-to-digital (ADCs) converters are the most widely used devices for information securing to translate the analog signals to digital numbers so microcontroller can read them easily. There are many ADC converters like ADC0801, ADC0802, ADC0803, ADC0804 and ADC080. In this article we are going to discuss about the ADC0804 converter.

ADC0804 is a very commonly used 8-bit analog to digital converter. It works with 0V to 5V analog input voltage. It has single analog input and 8-digital outputs. Conversion time is another major factor in judging an ADC, in ADC0804 conversion time varies depending on the clocking signals applied to CLK R and CLK IN pins, but it cannot be faster than 110 μs.

ADC0804Pin Description of ADC804:

Pin 1: It is a chip select pin and activates ADC, active low
Pin 2: It is an input pin; high to low pulse brings the data from internal registers to the output pins after conversion
Pin 3: It is an input pin; low to high pulse is given to start the conversion
Pin 4: It is a clock input pin, to give external clock
Pin 5: It is an output pin, goes low when conversion is complete
Pin 6: Analog non-inverting input
Pin 7: Analog inverting input, it’s normally ground
Pin 8: Ground (0V)
Pin 9: It is an input pin, sets the reference voltage for analog input
Pin 10: Ground (0V)
Pin 11 – Pin 18: It is an 8-bit digital output pins
Pin 19: Is used with clock IN pin when internal clock source is used
Pin 20: Supply voltage; 5V

Features of ADC0804:

  • 0V to 5V analog input voltage range with single 5V supply
  • Compatible with microcontrollers, access time is 135 ns
  • Easy interface to all microprocessors
  • Logic inputs and outputs meet both MOS and TTL voltage level specifications
  • Works with 2.5V (LM336) voltage reference
  • On-chip clock generator
  • No zero adjust required
  • 0.3[Prime] standard width 20-pin DIP package
  • Operates ratio metrically or with 5 VDC, 2.5 VDC, or analog span adjusted voltage reference
  • Differential analog voltage inputs

It is an 8-bit converter with 5V of power supply. It can take only one analog signal as input. The digital output varies from 0-255. ADC needs a clock to operate. The time taken to convert the analog to digital value depends on the clock source. An external clock can be given to CLK IN. Pin2 is the input pin – High to low pulse brings the data from internal register to the output pins after conversion. Pin3 is a Write – Low to high pulse is given to external clock.

Application:

From the simple circuit, pin 1 of ADC is connected to GND where pin4 is connected to GND through a capacitor; pin 2, 3 and 5 of ADC is connected to 13, 14 and 15 pin of microcontroller. Pin 8 and 10 are shorted and connected to GND, 19 pin of ADC is to 4th pin through resistor 10k. Pin 11 to 18 of ADC are connected to 1 to 8 pins of microcontroller which belongs to port1.

ADC0804 CircuitWhen the logic high is applied to CS and RD, input has been clocked through the 8-bit shift register, completing the specific absorption rate (SAR) search, on the next clock pulse; the digital word is transferred to the tri-state output. The output of the interrupt is inverted to provide an INTR output that is high during conversion and low when the conversion is completed. When a low is at both CS and RD, an output is applied to the DB0 through DB7 outputs and the interrupt is reset. When either the CS or RD inputs return to a high state, the DB0 through DB7 outputs are disabled (returned to the high-impedance state). Thus depending on the logic the voltage various from 0 to 5V which is transformed to digital value of 8 bit resolution, being fed as an input to the microcontroller port 1.

ADC0804 Component Used Projects
ADC0808 Component Used Projects

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